Sensors are utilized to indicate the value of a measured parameter, such as position, temperature, pressure, rotation, velocity, and the like, by providing an output signal. As an example, a linear position sensor can indicate the position of a movable mass. For example, such linear position sensors can indicate the position of a movable machine element, the location of a movable piston in a cylinder, or the level of a movable liquid in a container.
A magnetostrictive linear position transducer is a type of linear position sensor where a phenomenon known as magnetostriction is utilized to determine the position of the movable mass. In particular, in one such sensor, a control circuit transmits an interrogation signal along a wire that is adjacent the length of a waveguide, the waveguide being made from a magnetostrictive material. A magnet is movable along the waveguide and is connected to or otherwise follows the movable mass. When the interrogation signal reaches the location of the magnet, a magnetostrictive effect is created in the waveguide causing a strain wave and a electromagnetic wave to be generated and to travel along the waveguide. A detector is provided near the end of the waveguide to detect when one of these waves has reached the end of the waveguide and to cause an electrical return signal to then be generated.
A circuit or processor can then compute the amount of time elapsed between the sending of the interrogation signal and the generation of the return signal. The amount of time computed is directly proportional to the distance of the magnet from the coil end of the waveguide. The speed at which the magnetostrictive wave will travel along the waveguide is known (and is a function of the material of the waveguide). Accordingly, multiplying the time computed by the known magnetostrictive speed constant will result in the distance of the magnet along the waveguide. The distance can then be provided as an absolute value in an analog or digital format to a control device, such as to a programmable controller or the like.
However, certain control systems are configured to be used with sensors that provide incremental signals, such as quadrature signals for example. A linear encoder is one sensor device that typically provides a quadrature output signal which indicates the change in position of the movable element, rather than the absolute position of that element. In particular, such a device can provide a pair of square wave signals which are approximately ninety degrees out of phase. Each rising and falling edge of these two square wave signals represents an increase or decrease in position. The quadrature signals are provided to a control system, and a counter in the control system can keep a running tally of the increases and decreases, so that it continually has knowledge of the position of the movable element. The counter can increase or decrease the count according to the rising and falling edges of the two signals (the A and B signals). Whether the A signal leads or lags the B signal indicates whether the position is increasing or decreasing and whether the counter should increase or decrease its count. Using the position count, the control system can then control the process or machine at hand.
While many magnetostrictive position sensors provide digital or analog output signals, it is desirable to provide a magnetostrictive position sensor that provides a quadrature output such that it can be utilized with those control devices and control systems that require such signal formats as inputs. In particular, it is desirable to provide such a quadrature output sensor and other quadrature output devices that are accurate, that are low in cost, that provide selectable or programmable features, that minimize control circuit components, that indicate when errors have occurred, that provide an output representing a recent measurement, that account for waveguide variance, and/or that are easily modified as desired. Moreover, it is desirable to provide a quadrature output linear position sensors and other sensors that are easily calibrated.